The invention relates to a method for potting a populated or equipped assembly or module using an anti-vibration, thixotropic potting or embedding compound, a populated assembly potted according to the method and a controller having a populated assembly.
It is already known for populated assemblies to be potted by using an anti-vibration, thixotropic potting compound based on silicone (so-called Silgel). Silgel has a characteristic, which is referred to as thixotropic, of liquefying under the influence of a metering pressure used during the potting process, and of solidifying once again after emerging from the metering system. The potting process results in protection for electrical contacts, such as bonding wire connections or electrically conductive bonds in the populated assembly, against high vibration loads (of up to about 40 g) which occur, for example, in motor vehicles.
German Patent DE 197 12 842 C1, corresponding to U.S. application Ser. No. 09/407,255, filed Sep. 27, 1999, discloses a controller for a motor vehicle. That controller has an electrical circuit which is mounted on a baseplate and with which contact is made by a flexible printed circuit board that is routed to the circuit. The flexible printed circuit board is at the same time used as an electrical housing bushing.
An article entitled xe2x80x9cKapselung von Hybridschaltungenxe2x80x9d [Encapsulation of Hybrid Circuits], by G. Schwarz, in Elektronik Produktion and Prxc3xcftechnik [Electronics Production and Test Technology], July/August 1984, pages 419 to 421 describes the use of epoxy resin for the encapsulation of hybrid circuits. In that case, either the entire hybrid circuit is sheathed with epoxy resin, or individual components in the circuit are potted separately.
German Published, Non-Prosecuted Patent Application DE 195 15 187 A1 describes a chip cover for completely or partially covering electrical, electronic or optoelectronic components on a chip. A particular feature of the chip cover is that the cover has a pigmentation agent added to it.
German Patent DE 44 07 810 C2 describes a circuit configuration in which silicone potting is used, that electrically insulates the circuit layout and mechanically protects it against destruction. In that case the potting compound is also used as a cushioning element to compensate for pressures in a version of the circuit configuration using pressure contacts.
It is accordingly an object of the invention to provide a method for potting a populated assembly using an anti-vibration potting compound, a populated assembly and a controller having a populated assembly, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods, assemblies and devices of this general type, in which the method permits an electrical circuit with which a flexible printed circuit board makes contact to be potted by using a thixotropic potting compound, in a cost-saving manner and with reliable processes and in which the populated assembly can be produced cost-effectively and is resistant to vibration.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for partial potting of a populated assembly, which comprises providing a baseplate; mounting a substrate on the baseplate; equipping the substrate with electrical components; guiding or positioning a flexible printed circuit board relative to the substrate; establishing electrical contact between the printed circuit board and the equipped substrate through lead wires mounted at corresponding contact points on the printed circuit board; enveloping the components disposed on the substrate with an anti-vibration, thixotropic potting compound; and enveloping the contact points of the lead wires on the printed circuit board with the anti-vibration, thixotropic potting compound, in a separate process step.
With the objects of the invention in view, there is also provided, a populated assembly, comprising a baseplate; a substrate mounted on the baseplate; electrical components mounted on the substrate; a flexible printed circuit board positioned or guided relative to the substrate, the printed circuit board having contact points; corresponding lead wires mounted at the contact points and establishing electrical contact between the printed circuit board and the substrate having the electrical components; a first potting structure applied over the substrate and formed of thixotropic potting compound; and a second potting structure applied over the contact points separately from the first potting structure, the second potting structure likewise formed of thixotropic potting compound.
Trials carried out for the purposes of the invention have shown that, with such a circuit layout, there are difficulties in causing anti-vibration potting using a thixotropic potting compound to be applied to the substrate and to the contact points of the conductor wires on the flexible printed circuit board in one process step. If the potting compound is applied to the substrate with a relatively high thixotropic level, that has to be done by using a high metering pressure, so that the potting compound assumes a sufficiently low viscosity during the metering process, in order to allow it to reach both the electrical components and the contact points. The high metering pressure may damage bonding wire connections. Furthermore, due to the high thixotropic level, it is possible for difficulties to occur in which the potting compound solidifies too rapidly so that the bonding wire connections are then not completely sheathed, thus resulting in inadequate vibration protection. If a potting compound having a relatively low thixotropic level is used, it is admittedly possible to avoid the problems mentioned above, but the potting compound, which is a relatively thin liquid in that case, flows to a pronounced extent on the flexible printed circuit board. The flowing results in a large amount of potting compound being required. If the flexible printed circuit board is used as a housing bushing, it is furthermore a disadvantage that a housing having a large base area is required in order to prevent the potting compound from spreading into the seal region, where it may cause leaks later. Furthermore, for process reliability reasons, a comparatively large base-area tolerance range must also be provided for the potting compound to run over, since it has been found that the potting compound (for example thixotropic Silgel) as supplied by the manufacturer has considerable viscosity variations. Overall, it has been found that metering of the thixotropic potting compound using a sufficiently reliable process can only be achieved, within one process step, if at all, by taking highly costly additional measures (for example by adding barriers to prevent the potting compound from running) and/or by accepting large housing dimensions.
Those difficulties are overcome by the process according to the invention of potting the assembly in (at least) two process steps. A comparatively small amount of thixotropic potting compound is required. It is furthermore advantageous that considerably less space (that is to say baseplate area) need be provided for the potting compound to flow over, since the flow region is considerably reduced in size as the surface tension has a more pronounced effect when the amount of potting compound is reduced. This effect is particularly evident when potting the contact points.
In principle, the two process steps may be carried out by using potting compounds with different thixotropy levels. However, in accordance with another mode of the invention, the same potting compound with the same thixotropy level is preferably used for potting the electrical components and for potting the contact points since, in this case, the overall process can be carried out more cost effectively by using only one potting system.
In accordance with a first mode of the invention, the substrate, with the electrical components mounted thereon, is potted over the entire surface. This results in the maximum process speed.
In accordance with a second mode of the invention, at least some of the electrical components mounted on the substrate are provided with individual potting. The advantage thereof is that this requires even less potting compound than the first variant, which minimizes the material costs.
In accordance with a further mode of the invention, particularly with the first mode, it is preferable for the substrate to be disposed in a depression zone in the baseplate, or for the baseplate to have a trench structure surrounding the substrate. In both cases, potting compound going beyond the edge of the substrate can be caught in the depression zone or the trench structure.
In accordance with an added mode of the invention, with regard to the second process step which is associated with the potting of the contact points, individual contact points may be potted either individually or else jointly.
With the objects of the invention in view, there is additionally provided a controller for installation in a motor vehicle transmission or in a motor vehicle engine, comprising the populated assembly.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for potting a populated assembly using an anti-vibration potting compound, a populated assembly and a controller having a populated assembly, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.